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Key requirements of engines for vehicles are large output power and high efficiency, low emission as well as small size and light weight. Hydrogen combustion engines with direct injection have the characteristics to meet these factors. Tokyo City University, former Musashi Institute of Technology, has studied hydrogen fueled engines with direct injection since 1971. The key technology in the development of hydrogen fueled engines is the hydrogen injector for direct injection with the features such as high injection rate, high response and no hydrogen gas leakage from the needle valve of the hydrogen injector. A common-rail type system to actuate the needle valves of the high pressure hydrogen injectors was intentionally applied to fulfill good performances such as large injection rate, high response and no hydrogen gas leakage.

A statistical measurement of cyclic variation in combustion of a spark ignition engine generated by changes of mixture conditions in the cylinder was compared with the actual measurement of variation of flow velocity and mixture ratio. From these statistical data plus further analytical study, the correlations between the combustion speed, flow velocity, and mixture ratio were shown. These correlations explain the causing reason of combustion variation-it increases with the increase of combustion period. Regarding cyclic variation of combustion, although every possible factor influencing the variation of combustion was eliminated there still remains the variation caused by the variation of mean flow velocity in the cylinder. Thus, to decrease the combustion variation, it is fundamentally necessary to raise the combustion speed and decrease the variation of mean flow velocity in the cylinder.